A commonly used type of lighting installation includes a plurality of fluorescent lamps supplied with A.C. power through one or more electromagnetic ballasts. One ballast often serve several fluorescent lamps, and a complete installation may include a number of ballasts. Any such lighting installation comprises an inductive load, for which the overall harmonic distortion may be appreciable. As in any electrical system there may be load line faults or short circuits; moreover, in some instances any such installation may receive a surge of electrical power at an excessive voltage or a power input at an undesirably low voltage. All of these operational variations create problems for the inductive lighting installation; harmonic distortion, in particular can present substantial difficulties for other equipment (e.g., computers, calculators, and other office or technical apparatus) supplied from the same A.C. power source.
These problems associated with inductive lighting loads are exacerbated if a power reduction control is employed to reduce power consumption or if a light output control (dimmer) is superimposed on the installation. Previously known power reduction controls have a tendency to increase the overall distortion reflected back into the system. Indeed, harmonic distortion usually increases with each incremental stage of additional power reduction. Similarly, harmonic distortion increases result from successive stages of dimming control in any system utilized to regulate the light output of fluorescent lamps or like lighting installations. The distortion problems vary for different types of ballasts, but they are always present; for commercial purposes, a power reduction system should be able to perform effectively with several different ballast types.